The present invention pertains to arrangements relating to conductor carriers, e.g. printed circuit boards, for reducing crosstalk between densely packed conductors. The present invention also relates to methods for the manufacture of conductor carriers that include said arrangements.
In present times, so-called printed circuit boards or component carriers that can be used to implement different types of circuitry in a beneficial manner are used in practically all modern-day electronic equipment. These printed circuit boards have many advantages. For instance, they can be produced easily and can be arranged in the equipment in a readily perceived and space-saving manner, in addition to being easily exchanged. A printed circuit board is comprised of a carrying base part on which there is applied a non-conductive dielectric material on which there are disposed components that are interconnected by thin conductors provided in or on the layer of non-conductive material. The dielectric used is preferably a plastic-based material, wherewith the dielectric index xcex5r can be varied by including different amounts of additives in said material.
Present-day development trends, e.g. within mobile telephony, lean towards both ever smaller system solutions and, at the same time, towards ever greater transmission frequencies. This results in ever greater demands on the design and dimensioning of the printed circuit boards, wherewith a constantly increasing problem in this respect is one of minimising crosstalk between the more and more densely packed conductors on a printed circuit board.
Crosstalk is an undesirable effect between two conductors due to the electromagnetic field that occurs as a result of the signal currents in respective conductors. This can be understood as a parasitic capacitive and inductive coupling between the conductors. Crosstalk can be considered to be proportional to a coupling coefficient K, defined as
K=Cm/C=Lm/L.
In this relationship, Cm and Lm signify the mutual capacitance and the mutual inductance between two parallel conductors and C and L signify respectively the self-capacitance and the self-inductance of the conductors. In an equivalence interpretation, it suffices either to consider solely the case of a capacitive coupling or solely the case of an inductive coupling. Consequently, the following discussion is concerned solely with the capacitive coupling. The so-defined coupling coefficient is a measurement of the magnitude of an undesired parasitic capacitance in comparison with the self-capacitance of the conductor (c.f. also FIG. 1).
A reduction in crosstalk effects is synonymous with minimising the value of the coupling coefficient K. In order to obtain a high resistance in the space between two conductors, the mutual capacitance Cm between the conductors must be as small as possible. On the other hand, the conductor capacitance C between conductors and the earth plane shall be high. As will be evident from the definition of the coupling coefficient, it is also necessary for the relationship or ratio between the mutual capacitance Cm and the conductor capacitance C to be small.
It is apparent from the general definition of a capacitance
C=xcex50xcex5r(A∫EdA/s∫Eds)
of an arbitrary electrode arrangement that its size is determined essentially by the conditions in the space between two electrodes. More specifically, this includes both the geometrical dimensions, particularly the spacing of the electrodes, and the intermediate dielectric material, which is characterised by its dielectric index xcex5r.
Hitherto, the simplest method of minimising crosstalk between two conductors has therefore been to increase the distance therebetween. Typical conductor spacings in this respect can be from three to four times the width of the conductors and can reach magnitudes in the order of about 600 xcexcm-800 xcexcm. However, this means unacceptable limitations in the design of an optimal printed circuit board design that shall accommodate the largest possible number of components within a small area. Crosstalk can also be minimised by using in the space between the conductors a dielectric material that has a low dielectric index.
An arrangement for minimising crosstalk that occurs between conductors on a printed circuit board is described in EP 0 354 671 A1. Posts of dielectric material that carry metallic conductors are disposed on a metallic base plane. These vertical posts are mutually separated by air-filled spaces that extend right down to the base plane. This results in a reduced capacitive coupling between the conductors and thus also in a reduction in crosstalk, among other things.
JP 3-041 803 A teaches another arrangement for reducing crosstalk between conductors on a printed circuit board. This is achieved by including between the conductors an arrangement that consists of a dielectric material that has a lower dielectric index than the other dielectric material.
The present invention addresses the problem of reducing crosstalk between densely packed conductors on a printed circuit board, for instance.
A first object of the invention is to reduce crosstalk effects between two conductors on a printed circuit board.
Another object of the present invention is to provide a printed circuit board that includes an arrangement for reducing crosstalk effects between two conductors, wherewith functioning of said arrangement is essentially independent of the distance between said two conductors and of the material present therebetween.
A further object of the present invention is to provide a method of manufacturing the inventive arrangement.
These objects are achieved in accordance with the invention, by providing a high capacitance in a space between each of the conductors and the earth plane, if possible a narrow space, and therewith bind the electric field to said earth plane substantially within said space, so as to prevent the leakage of field lines to the co-lateral conductors such that crosstalk between said conductors is prevented.
The invention is based on the realisation that this increase in capacitance can be achieved by disposing in the space immediately beneath a signal conductor an arrangement that consists of a thinner dielectric layer and/or a dielectric material that has a higher dielectric index xcex5r than the remaining dielectric material.
In one advantageous embodiment, the arrangement is narrower than the width of the conductor and is placed symmetrically thereto.
The inventive arrangement and the methods of producing printed circuit boards that include said arrangement have the characteristic features disclosed more specifically in respective Claims.
A first advantage afforded by the inventive arrangement is that crosstalk between conductors is reduced.
Another important advantage afforded by the invention is that this effect is achieved essentially regardless of the geometry of the printed circuit board or of its material properties in the space between said conductors. This enables, for instance, appropriate selection of the dielectric material in this space. A material that has a low electric index xcex5r enables crosstalk to be further reduced.
This also means that said effect is independent of the design of the printed circuit board and does not restrict dimensioning of the board and the arrangement of conductors thereon. Thus, the conductors can be packed more densely on the printed circuit board without limitations due to crosstalk effects, so that the distance between two conductors may be of the same order of magnitude as the width of the conductors.
A further advantage afforded by the invention is that the increase in capacitance that reduces crosstalk can be achieved in two different ways, which may be used either individually or, preferably, in co-operation with one another.